Pressure sensitive adhesives are widely used for a multitude of purposes, such as various sealing and attachment purposes. Pressure sensitive adhesives may be formed as supported systems, for instance as an adhesive layer supported by a foundation material, which foundation may be a flexible support or carrier, or a rigid body. Pressure sensitive adhesives may also be formed as double-faced supported systems, for instance with a flexible or rigid carrier faced on both sides with an adhesive layer. Such a carrier may itself be nonadhesive, although for certain applications, discussed in more detail below, the carrier supporting the two adhesive layers has itself some pressure sensitive adhesive qualities. Pressure sensitive adhesive layers vary widely as to thickness, from a fraction of a rail (10.sup.-3 inch) to much greater thickness.
Pressure sensitive adhesives formed substantially from acrylate copolymers are well known in the art. For instance, certain acrylate copolymers which are both normally tacky and yet possess a reasonable degree of internal strength are disclosed in U.S. Pat. No. Re 24,906 (Ulrich) which reissued from original U.S. Pat. No. 2,884,126 in December 1960, incorporated hereinto by reference. Such copolymers are comprised of certain non-tertiary acrylic esters copolymerized with monomers having polar groups, for instance acrylic acid, methacrylic acid, acrylamide methacrylamide, itaconic acid, and the like. Such non-tertiary acrylic esters generally are acrylic acid esters of non-tertiary alkyl alcohols, such alcohols having up to 14 carbons, and further having an average of from 4 to 12 carbons. Small amounts of other monomers may also be incorporated into such polymers. Such adhesive polymers, referred to herein at times as acrylic ester polymer type or acrylic ester polymer(s), are commonly formed as supported systems on a flexible support sheet or tape.
Photopolymerization of pressure sensitive adhesive materials of the acrylic ester polymer type is generally known. While such acrylic ester polymers may be prepared by solution or emulsion polymerization techniques, there are manufacturing disadvantages in preparing the polymers in such fashion. If solution polymerization is used, the solvent must be removed, typically by drying it off at elevated temperatures, which involves the use of significant processing equipment and may create environment and safety hazards. Emulsion polymerization also requires the removal of a liquid. The acrylic ester polymers must be separated from the emulsion liquid(s) used, which often would be primarily water. The emulsified acrylic ester polymers may be isolated by precipitation, rather than by drying off the water, which may reduce the processing equipment required. In either instance, however, the emulsion polymerization product will contain some amount of the surfactant used in the polymerization which may be detrimental to the product's end use, and the additional polymer-isolation steps are not entirely avoided.
Photopolymerization of the monomers neat, without any diluent that requires removal after the polymerization, provides processing advantages in the manufacture of pressure sensitive adhesives. For instance, Belgium Patent No. 675,420, published in May of 1966, discloses a process for the fabrication of adhesive tapes by polymerizing the monomers directly on the carrier using photopolymerization through exposure of the monomers, containing a photoinitiator, to ultraviolet radiation. The liquid acrylic monomer(s), which may be thickened, are applied to the carrier using a doctor blade or roller coating, or by spraying, and then polymerized, avoiding any post-polymerization removal of diluent or other extraneous material. Exemplified is a typical pressure sensitive formulation, a mixture of ethyl hexyl acrylate and acrylic aid, that is coated onto a cellulose acetate sheet and is passed into a tunnel, from which oxygen has been excluded, the upper wall of which is formed of quartz glass, and is irradiated with a source of ultraviolet radiation through such upper wall. The source of ultraviolet radiation used is a high pressure mercury vapor lamp.
It is also known to prepare a pressure sensitive adhesive tape by first forming a prepolymer of at least some of the monomers to be used, and then forming a coating of such prepolymer of the desired thickness before completion of the polymerization. For instance, U.S. Pat. No. 3,729,338 (Lehmann et al.) discloses the preparation of a low molecular weight, spreadable composition which is applied to the support material, to which composition may be added a small amount of catalyst and/or poly-functional cross-linking monomer, prior to the completion of the polymerization by heat curing. Such low molecular weight polymer, or prepolymer, is thicker or more viscous than the liquid monomers and hence is more easily applied as a coating to the support material before polymerization is completed.
Pressure sensitive adhesive tapes that are both relatively thick and are comprised of layers of acrylate esters have been prepared using photopolymerization techniques. For instance, U.S. Pat. No. 4,223,067 (Levens) describes the preparation of a pressure sensitive adhesive tape that may be from 0.2 to 1.0 mm thick. Such tape is comprised of a pressure sensitive adhesive matrix which further contains glass microbubbles, which optionally may have one or both of its sides coated with an untilled pressure sensitive adhesive. The photopolymerization is accomplished by exposure to ultraviolet BL and BLB lamps. Where a laminate of both filled and untilled layers was prepared, the separate polymerized layers were laminated together using a pressure roll.
The use of BL and BLB lamps in particular for the photopolymerization of acrylic ester polymer type pressure sensitive adhesives is discussed in detail U.S. Pat. No. 4,181,752 (Martens et al.). Such lamps emit ultraviolet radiation that peaks at about 351 nm, and the energy emitted at ultraviolet radiation wavelengths shorter than 300 nm is less than 10 percent of that in the 300 to 400 nm range, and at a distance of about 3 inches have a light intensity in the 300-400 nm wavelength band of about 0.337 milliwatts/cm.sup.2. An unduly slow polymerization rate is noted as being disadvantageous to the manufacture of the pressure sensitive adhesive.
The preparation of acrylic ester polymer type pressure sensitive adhesives at least partially by photopolymerization using a source of ultraviolet radiation having a spectral peak at about 310 nm is disclosed in U.S. Pat. No. 5,183,833, Dennis K. Fisher and Brian J. Briddell, issued on Feb. 2, 1993, incorporated hereinto by reference. Using UV-B lamps, such as 20 watt Westinghouse FS-20 lamps, a pressure sensitive adhesive system having an inner filled layer of about 34 mils thickness sandwiched between adhesive layers of 2 to 3 mils thickness was formed. Such pressure sensitive adhesive system had excellent adhesion characteristics, for instance ratings of 61.7 lb in the pluck test, 37.6 lb in the initial breakaway peel test, and 19.1 1b in the initial continuing peel test. A steel panel coated with an exterior finish automotive paint was one of the surfaces used in such tests.
The automotive field uses pressure sensitive adhesives for the application of exterior trim. The adhesive characteristics of a pressure sensitive adhesive is in part dependent on the surface to which it is applied. As the nature of commercial automotive coating materials evolves, the adhesion performance of the pressure sensitive adhesives can diminish. In the instance of new "silane-modified" acrylic/melamine coatings (described below), the reduced adhesion of otherwise superior pressure sensitive adhesives products has been seen.
It is an object of the present invention to provide a fortified pressure sensitive adhesive. It is an object of the present invention to provide a pressure sensitive adhesive having improved adhesion to silane-modified acrylic/melamine coatings. It is an object of the present invention to provide a process for preparing a pressure sensitive adhesive having improved adhesion characteristics, particularly as to its adhesion to silane-modified acrylic/melamine coatings. It is an object of the present invention to provide such a pressure sensitive adhesive, and process for the preparation of a pressure sensitive adhesive, particularly a pressure sensitive adhesive of a relatively thick pressure sensitive adhesive system. These and other objects of the present invention are described in more detail below.